In recent years, the amount of information that can be transmitted (transmission capacity) via one optical fiber has kept expanding owing to an increase in number of wavelength channels and a speedup of a modulation speed of an optical signal, but has almost reached capacity. This is because a wavelength bandwidth of an optical fiber amplifier that can be used for optical transmission has been almost used up. Under such circumstances, in order to further expand the transmission capacity of the optical fiber, it is necessary to enhance use efficiency of the frequency bandwidth by devising a signal modulation format to include a large number of optical signals in a limited frequency bandwidth.
In the world of radio communications, a multilevel modulation technology that has become widespread since 1960s allows transmission at such high efficiency that frequency use efficiency exceeds 10 (bit/s/Hz/sector). There have conventionally been many studies of multilevel modulation which is regarded as promising also in the field of optical fiber transmission. For example, “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration”, R. A. Griffin, et al., OFC 2002, paper PD-FD6, 2002 discloses a technology of quadrature phase shift keying (QPSK) for performing four-level phase modulation. In addition, “Proposal and Demonstration of 10-Gsymbol/sec 16-ary (40 Gbit/s) Optical Modulation/Demodulation Scheme”, Kenro Sekine, et al., paper We3.4.5, ECOC 2004, 2004 discloses a technology of sixteen-level phase and amplitude modulation that is a combination of four-level amplitude modulation and the four-level phase modulation.
FIGS. 1A to 1D are explanatory diagrams indicating characteristics of various conventional modulation formats that can be applied to the optical transmission.
In the examples of FIGS. 1A to 1D, signal points of optical modulation (display of complex information on an optical field at a decision timing for a signal) are plotted in a phase plane (on an IQ plane).
FIG. 1A is an explanatory diagram for a signal point on the phase plane, and each of signal points is equivalently displayed by a complex Cartesian coordinate on the IQ plane or polar coordinates including an amplitude r(n) and a phase φ(n).
FIG. 1B illustrates a signal example of the four-level phase modulation (QPSK) in which four values (0, π/2, π, and −π/2) are used as the phase angle φ(n) and two-bit information (00, 01, 11, and 10) is transmitted per symbol.
FIG. 1C illustrates a signal example of sixteen-level quadrature amplitude modulation (16 quadrature amplitude modulation (16QAM)) widely used in radio communications. The 16QAM, in which signal points are arranged in lattice, allows four-bit information to be transmitted per symbol. In the example of FIG. 1C, the Q-axis coordinate represents a value of upper two bits (10xx, 11xx, 01xx, and 00xx), and the I-axis coordinate represents a value of lower two bits (xx10, xx11, xx01, and xx00).
In a signal constellation of the 16QAM, it is possible to increase a distance between the signal points, which enhances the reception sensitivity, and in optical communications, the quadrature amplitude modulation can be realized by using a coherent optical receiver.
For example, “1 Gsymbol/s, 64QAM Coherent Optical Transmission over 150 km with a Spectral Efficiency of 3 Bit/s/Hz”, J. Hongou, K. Kasai, M. Yoshida and M. Nakazawa, in Proc. Optical Fiber Communication Conf. (OFC/NFOFEC), Anaheim, Calif., March 2007, paper OMP3. discloses an experimental example of transmission/reception using a 64QAM signal. The coherent optical receiver represents a receiver that uses a local light source disposed within the receiver in order to detect the phase angle of the optical signal.
FIG. 1D illustrates a signal example of a sixteen-level phase and amplitude modulation format (16APSK format) in which the same number of signal points are arranged radiately in concentric circular shapes on the IQ plane.
Here, description is made of a coherent reception format which is one of conventional technologies for an optical multilevel receiver, for example, a coherent optical field receiver disclosed in “Coherent detection method using DSP to demodulate signal and for subsequent equalisation of propagation impairments”, M. G. Taylor, paper We4. P. 111, ECOC 2003, 2003.
FIG. 2 is a configuration diagram of a coherent optical field receiver of a polarization diversity type, and the coherent optical field receiver of a polarization diversity type simultaneously receives information on two polarizations of the optical signal. An input optical signal 101 transmitted through an optical fiber transmission line is split into a horizontal (S) polarization component 105 and a vertical (P) polarization component 106 by a polarization beam splitter 102-1. The S polarization component 105 and the P polarization component 106 obtained by the splitting are input to the coherent optical field receiver 100-1 and the coherent optical field receiver 100-2, respectively.
In the coherent optical field receiver 100-1, a local laser source 103 having a wavelength substantially the same as the input optical signal 101 is used as a reference of an optical phase. local light 104-1 output from the local laser source 103 is split into two beams of local light 104-2 and local light 104-3 by a polarization beam splitter 102-2. The local light 104-2 and the local light 104-3 obtained by the splitting are input to the coherent optical field receiver 100-1 and the coherent optical field receiver 100-2, respectively.
In the coherent optical field receiver 100-1, an optical phase diversity circuit 107 combines the S polarization component 105 of an optical multilevel signal and the local light 104-2 which have been input with each other. The optical phase diversity circuit 107 generates an I (inphase) component output light 108 extracted from an inphase component of the local light 104-2 and the S polarization component 105 of the optical multilevel signal that have been combined with each other, and a Q (quadrature) component output light 109 extracted from a quadrature component of the local light 104-2 and the S polarization component 105 of the optical multilevel signal that have been combined with each other. The I component output light 108 and the Q component output light that have been generated are received by balanced optical receivers 110-1 and 110-2, respectively, the I component output light 108 and the Q component output light that have been received are converted into electrical signals. Then, the two electrical signals obtained by the conversion are time-sampled by A/D converters 111-1 and 111-2 to generate digitized output signals 112-1 and 112-2, respectively.
In the following description, as illustrated in FIG. 1A, the optical field of the received input optical signal 101 is represented as r(n)exp(jφ(n)). Here, the optical field of the local light 104-2 and the local light 104-3 is assumed to be 1 (originally including an optical frequency component, which is omitted). Further, “r” represents an amplitude of the optical field, “φ” represents a phase of the optical field, and “n” represents a sampling timing.
The local light 104 actually includes random phase noise and a slight difference frequency component with respect to signal light, but the phase noise and the difference frequency component, which exhibit temporally slow phase rotation, are eliminated by a digital signal processing and therefore ignored.
The balanced optical detector 110-1 and the balanced optical detector 110-2 perform homodyne detection on the input optical signal 101 that has been input by using the local light 104-2, and output an inphase component and a quadrature component, respectively, of the optical field of the input optical signal 101 by taking the local light as a reference.
Therefore, the output signal 112-1 from the A/D converter 111-1 is represented by I(n)=r(n)cos(φ(n)), and the output signal 112-2 from the A/D converter 111-2 is represented by Q(n)=r(n)sin(φ(n)). However, in order to simplify a formula, constants including a conversion factor are all set to “1”.
In the coherent optical field receiver, an optical multilevel signal can be received because all information pieces represented by an optical field r(n)exp(φ(n)) (here, I component and Q component) are easily obtained from the received input optical signal 101.
A digital operation circuit 113, which is a complex field operation circuit, gives an inverse function to linear degradation (for example, chromatic dispersion) exerted upon the optical signal during transmission, to thereby enable cancellation of influences including the linear degradation substantially completely. Further, processings such as retiming and resampling are performed as necessary to output an inphase component 114-1 of an optical field signal and a quadrature component 114-2 of the optical field signal that have been subjected to the processings.
As described above, the coherent optical field receiver 100-1 can obtain field information on one polarization component (for example, S polarization component) of the input optical signal 101 that has been received, but needs to receive the P polarization component as well because a polarization state of the optical signal changes during the optical fiber transmission. Therefore, the coherent optical field receiver 100-2 receives the P polarization component of the input optical signal 101 in the same manner, and outputs the field information on the received P polarization component as an optical field signal 114-3 and an optical field signal 114-4.
A digital operation/symbol decision circuit 115 resolves the change of the polarization state by subjecting the I component and the Q component of the respective polarizations output from the digital operation circuit 113 to conversion of the polarization state of the optical signal (for example, conversion of a linear polarization into a circular polarization).
Subsequently, the digital operation/symbol decision circuit 115 decides which symbol has been transmitted with high precision in comparison with, for example, the signal constellation illustrated in FIG. 1C. A decision result thereof is output as a multilevel digital signal 116.
By using the coherent optical field receivers described above, it is possible to obtain all the field information pieces on the received signal, which allows even a complicated multilevel signal to be received.
Further, the coherent optical field receivers described above cause the digital operation circuit 113 to subject an input signal to a compensation processing using an inverse function to a propagator of the optical fiber transmission line, which can compensate the linear degradation due to the chromatic dispersion or the like logically completely. Further, the compensation processing is greatly advantageous in that no limitation is imposed upon a compensation amount. However, the digital operation circuit 113 that is small in size and high in speed and has signal processing performance equal to or higher than 10 Gbit/sec is not available on the market at present, and is still in a verification step for effects obtained by partial experiments.
FIG. 3A is an example of eight-level phase and amplitude modulation light (8APSK) in which eight signal points are arranged in concentric circular shapes with a four-level phase and a binary amplitude. FIG. 3B is a configuration diagram of a conventional optical multilevel signal receiver for receiving phase and amplitude modulation light disclosed in Sekine et al.
In optical modulation in which a phase component is evenly divided as in an 8APSK signal, differential coding is used for modulation of the phase component. In the example of FIG. 3B, two levels (one bit) in amplitude and four levels (two bits) in phase difference from the previous symbol, i.e., 0, π/2, π, and −π/2, are used for information transmission, and three-bit information is transmitted per symbol.
In the example of FIG. 3B, the 8APSK signal is used as the input optical signal 101. An optical splitter 120 splits the input optical signal 101 that has been input into three optical signals. Two of the optical signals obtained by the splitting are input to optical delay detectors 121-1 and 121-2, and the remaining one optical signal is input to an optical intensity detector 122.
The optical delay detectors 121-1 and 121-2 each include a first optical path that applies a delay of a symbol time T to the input signal and a second optical path that passes through a −π/4 phase shifter or a +π/4 phase shifter, and convert a phase modulation component into an optical intensity signal by causing the input optical signal 101 that has been input to interfere with a signal that had been received earlier by a timing T.
An output intensity of output light from the optical delay detector 121-1 that has passed through the +π/4 phase shifter becomes maximum when the phase difference between the reception symbol and the previous symbol is 0 or +π/2, and becomes minimum when the phase difference is −π/2 or π.
A binary decision circuit 123-1 receives the output light from the optical delay detector 121-1 via the balanced optical detector 110-1. A binary digital signal 124-1 for one bit is obtained by subjecting the received output light to a binary decision.
An output intensity of output light from the optical delay detector 121-2 that has passed through the −π/4 phase shifter becomes maximum when the phase difference between the reception symbol and the previous symbol is 0 or −π/2, and becomes minimum when the phase difference is π/2 or π.
A binary decision circuit 123-2 receives the output light from the optical delay detector 121-2 via the balanced optical detector 110-2. Another binary digital signal 124-2 for one bit included in the phase component is obtained by subjecting the received output light to the binary decision.
The optical intensity detector 122 converts an optical intensity of a received signal (square of an optical field amplitude) into an electrical signal. A binary digital signal 124-3 for one bit included in the amplitude component is obtained by causing a binary decision circuit 123-3 to subject the output of the electrical signal obtained by the conversion to the binary decision.
The optical multilevel signal receiver uses the optical delay detection, and hence a phase change of a light source or reception polarization dependence is reduced to a minimum, which makes a local oscillation light source unnecessary. Therefore, the optical multilevel signal receiver is applied to reception of an n-level phase modulation signal and an APSK signal having a radiate signal constellation up to sixteen-level.